The invention relates generally to the field of aperture antennas, and more specifically provides a means of reducing the maximum peak power density of an aperture antenna that occurs in the near field region relative to the average power density concentration at other ranges within the entire near field.
Microwave transmitting antennas of the aperture type operating at millimeter wavelengths have an equivalent aperture diameter of many wavelengths that defines a near field region extending as far as hundreds of meters. Applications that operate in the near field, such as Active Denial Technology (ADT), require antennas that produce power density characteristics that are compatible with the application requirements. There is a difficulty with applications that require a power density that lies between a minimum level, P1 and a maximum level P2, in that there exists a peak power density at a range in the near field commonly referred to as the Fresnel maximum that sets the limit at P2 and constricts the depth of ranges that will remain above the minimum level P1. The power density in the near field is calculated using scalar potential theory which is well understood by those skilled in the art. A plot of the power density on the boresight of a square aperture antenna with uniform illumination vs. range is shown in FIG. 1. The aperture is one meter square with a total illumination of 1-kW at a frequency of 100 GHz. The phase front at the aperture has zero curvature corresponding to an infinite focal length. The range of the near field boundary (RNFB) at which the field of the antenna transitions from the near field to the far field is typically approximated by the relation:
                    RNFB        ≅                              4            ⁢            A                                π            ⁢                                                  ⁢            λ                                              [                  Eq          .                                          ⁢          1                ]            Where, A is the area of the aperture in square meters and A is the wavelength in meters. The antenna in FIG. 1 has a RNFB of 403 meters.
In FIG. 1 the Fresnel peak is at a range of about 125 meters and amplitude of about 3250 W/m2. It is obvious that the maximum allowable peak power density of the system is determined by the Fresnel peak. This in turn determines the ranges over which the minimum required power density is available. Clearly, if the peak power density at the Fresnel maximum could be reduced without significant changes to the remainder of the power profile, then the transmitter power could be increased, and the operating range and performance of the system would increase.